Silicon segment programming method and apparatus

ABSTRACT

The present invention is a method and apparatus for programming a stack of segments wherein each segment includes a plurality of die which are interconnected through metal interconnects patterned on the surface of each segment Once the segments are arranged into a stack, the stack is connected to external circuits and each segment is addressed through control lines. Electrically conductive fuses on the segments are used as an interface between the control lines and the die. Segment level programming is performed on each segment by opening the conductive fuses on the segments in a predetermined pattern in order to route the control lines to each segment such that segments are uniquely addressed. After segment level programming, circuit board programming is performed so that any defective die found in the stack is logically replaced with replacement die in the stack. This is accomplished by connecting a set of metal switches between all the die and each of the control lines and by dispensing a conductive epoxy whisker between the control line for the defective die and the metal switch of the replacement die. When a subsequent attempt is made to address the defective die in the stack, the replacement die is accessed instead.

CROSS-REFERENCE TO RELATED PATENT APPLICATION

The present application is a continuation-in-part of application Ser.No. 08/265,081, entitled "Vertical Interconnect Process for SiliconSegments," filed on Jun. 23, 1994, and assigned to the assignee of thepresent application.

BACKGROUND OF THE INVENTION

The present invention relates to a method and apparatus for addressingintegrated circuit chips, and more particularly to a method andapparatus for addressing a stack of silicon segments.

For many years, electrical components such as transistors and integratedcircuits have been made using wafers of semiconductor material,including silicon and germanium. Integrated circuits have been providedon the wafer. Individual integrated circuits that are provided on thewafer are referred to as die, and include contact points called bondpads for external electrical connections. Typically, the die on thewafer are separated from one another by cutting the wafer alongboundaries defining the die. Once the die are cut from the wafer, theyare referred to as chips, and are packaged for use. In recent years, theproliferation of more powerful electronic systems has led to anincreased need for higher density integrated circuit packages.

One method for creating higher density packages attempts to createentire computer systems on a single wafer using wafer scale integration(WSI) techniques. WSI technology attempts to laterally wire together allthe die on a wafer using wires to interconnect the die. However, inorder to create the necessary interconnections between the die, manywires are required that are extremely thin and difficult to create.

A second method for creating higher density packages attempts to reducethe area required for placing the chips on a circuit board by physicallystacking the chips vertically. One chip stacking technique mountsindividual die on ceramic carriers, encapsulates both the die and thecarrier, stacks the carriers, and then mounts the stack on a printedcircuit board. In this technique, all the die in the stack areinterconnected by connecting the leads of the die to the printed circuitboard via metal pins. This method results in an unusually high pin counton the circuit board which reduces the reliability of the circuitrybecause the high pin count increases the possibility that one of themany pins may become disconnected from the board.

Another chip stacking method uses a more complex process to stack die.This method modifies individual chips by adding a pattern ofmetallization, called rerouting leads, to the surface of the wafer. Thererouting leads extend from bond pads on the chip to newly formed bondpads, and are arranged so that all the rerouting leads terminate on oneside of the modified chip. Each modified chip is then cut from thewafer, as shown by the dotted lines, and assembled into a stack. Afterthe leads of the chips are exposed, a layer of metallization is appliedto the leads along the side of the stack in order to electricallyconnect each of the modified chips in the stalk. The stack is thenmounted and connected to a substrate which is in turn connected toconventional circuitry.

The method of rerouting leads offers improvement in circuit density overprior methods, but is complex and expensive. In addition, the reroutingleads extend over five adjacent die which are destroyed when themodified chip is cut out of the wafer. In this method, five die aresacrificed for every chip that is modified.

Another method for creating higher density circuits creates stacks fromentire wafers, rather than individual chips, to form a wafer array. Insome devices, the wafers in the stack are electrically interconnectedusing solid vertical columns of metallic conductive feed-throughs, suchas copper. The use of solid feed-throughs to interconnect wafers maycause damage to the array due to differential thermal coefficients ofexpansion during thermal cycles. Furthermore, the process is costly andmakes the wafers difficult to separate for repairs.

Other methods also exist to interconnect stacks of wafers, as disclosedin, for example, U.S. Pat. No. 4,897,708 issued Jun. 30, 1990, and U.S.Pat. No. 4,954,875 issued Sep. 4, 1990. These methods provide each waferin the stack with coned-shaped through holes which expose bonding padson the wafers. The bond pads of the wafers in the stack are thenelectrically connected by either filling the through holes withelectrically conductive liquid, or inserting an electrically conductivecompliant material into the through holes, to provide a continuousvertical electrical connection between the wafers. While avoiding thedisadvantages of using solid vertical columns of metal to interconnectwafers, the use of electrically conductive liquids and conductivematerials requires special tooling to fill the through holes.Furthermore, for some applications, it may not be desirable to usestacks of entire wafers due to size constraints of the electricaldevice.

In integrated circuit packages, individual chips are accessed throughthe use of address lines, data lines, and control lines; collectivelycalled control lines. The address lines are divided into row and columnaddress lines which are controlled by a row address select line and acolumn address select line, respectively. To electrically connect anintegrated circuit package to a substrate, such as to a printed circuitboard for example, the control lines are extended from individual chipsin the integrated circuit packages to the circuit board via metaltraces. Since the addressing of chips is, in effect, hard wired once theintegrated circuit package is connected to a substrate, defective chipsare typically discarded before the chips are stacked and/or connected toa circuit board in order to save space and to avoid the difficulty andexpense associated with rerouting the control lines from defective chipsto functioning chips.

In parent application Ser. No. 08/265,081, which is herein incorporatedby reference, a vertical interconnect process (VIP) is disclosed whichprovides an improved method and apparatus for creating higher densitypackages. In VIP, a segment is formed by grouping a plurality ofadjacent die on a wafer. The plurality of die on a segment areinterconnected on the segment using one or more layers of metalinterconnects. The metal interconnects function not only to interconnectthe die, but also to provide segment bond pads, which serve as externalelectrical connection points. After the die are interconnected, eachsegment is cut from the wafer so as to have beveled edge walls. Segmentsare then placed on top of one another to form a stack of segments, asopposed to a stack of individual chips, and the segments areelectrically connected through the application of electricallyconductive epoxy along the beveled edges of the stack. The stack ofelectrically interconnected segments is then mounted to a circuit board.

Since a portion of the die on a wafer may not function and the defectivedie are not cut from the wafer and discarded, addressing the stack andthe die therein solely through the use of hard-wired control lines, asin prior art methods, is inadequate because a computer or the like mayattempt to access a defective die in the stack.

SUMMARY OF THE INVENTION

Accordingly, it is an object of the present invention to provide animproved method and apparatus for uniquely addressing chips, as well asstacks of segments.

The present invention is a method and apparatus for programming a stackof segments that provides an addressing scheme capable of uniquelyaddressing each segment in a stack as well as providing access to afunctioning die when an attempt is made to access a defective die in thestack. Each segment in the stack includes a plurality of die which areinterconnected through metal interconnects patterned on the surface ofeach segment. Once segments are arranged into a stack, external circuitsaccess the segments through control lines. Connected between all of thedie on each segment and the control lines are electrically conductivefuses. The segments, which are all located on different levels of thestack, are programmed by opening the conductive fuses in a predeterminedpattern on each die so that the control line associated with that levelof the stack is routed to all the die on that segment, thereby makingeach segment address in the stack unique.

After the stack is connected to the external circuits in a particularelectronic application, the stack is programmed so that a defective diein the stack is logically replaced with a replacement die in the stack.This is accomplished by connecting a set of metal switches between allthe die and each of the control lines. The control line for thedefective die is then routed to the replacement die by dispensingconductive epoxy between the replacement die's metal switch and thecontrol line for the defective die. When an attempt is made to addressthe defective die's location in the stack, the replacement die isaccessed instead.

Other objects, features and advantages of the present invention willbecome apparent from the following detailed description when taken inconjunction with the accompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

The accompanying drawings, which are incorporated in and form a part ofthis specification, illustrate embodiments of the invention and,together with the following detailed description, serve to explain theprinciples of the invention:

FIG. 1 is a cut-away perspective view illustrating a stack of segments.

FIG. 2 is a diagram illustrating an address signal extending to four dieon a segment through conductive fuses.

FIG. 3 is a diagram illustrating a conductive fuse of the presentinvention, and one preferred method of opening the conductive fuse usinga circuit.

FIGS. 4A and 4B are diagrams illustrating a second preferred embodimentof the conductive fuse, and a method of opening the conductive fuseusing a circuit of the present invention.

FIG. 5 is a diagram illustrating a third preferred embodiment of openinga conductive fuse using a laser.

FIG. 6 is a diagram illustrating a method of segment level programmingaccording to the present invention.

FIG. 7 is a diagram illustrating a stack that is subsurface mounted in acircuit board.

FIG. 8 is a perspective view illustrating a stack comprising five levelsof segments.

FIG. 9 is a top view of a stack and a circuit board showing how the dieon each segment are addressed by external circuits.

FIGS. 10A and 10B are top views of a stack and a circuit board showinghow a defective die in the stack is logically replaced with a functionaldie in the stack.

DETAILED DESCRIPTION OF THE INVENTION

Reference will now be made in detail to the preferred embodiments of theinvention, examples of which are illustrated in the accompanyingdrawings. While the invention will be described in conjunction with thepreferred embodiments, it will be understood that they are not intendedto limit the invention to those embodiments. On the contrary, theinvention is intended to cover alternatives, modifications andequivalents, which may be included within the spirit and scope of theinvention as defined by the appended claims.

Referring to FIG. 1, a cut-away perspective view is shown of a stack 10of the present invention. The stack 10 includes four segments, 12a, 12b,12c, and 12d (hereinafter segments 12) where each of the segments 12represent a different level in the stack 10. As disclosed in applicationSer. No. 08/265,081, the segments 12 each include four die 14 arrangedin a two-by-two matrix, as shown by the positional overlay of the fourdie 14 on each level of the stack.

Each of the die 14 on the segments 12 are interconnected using multiplelayers of die interconnect circuitry. As shown in FIG. 1, the dieinterconnect circuitry on each segment 12 includes two layers of goldinterconnects 34 and 40 which function to communicate power andelectrical signals between die bond pads 20 as well as to selectedsegment bond pads 22. The die 14 first are insulated with a polyimidelayer 30 that covers the entire surface 24 of the segments 12 except fora first set of holes 32 in the polyimide layer 30 which expose thesurface of the die bond pads 20. The first layer of gold interconnects34 is supported by the polyimide layer 30 and makes electricalconnection to the die bond pads 20 through the first set of holes 32.

The first layer of gold interconnects 34 is insulated with a secondpolyimide layer 36, and a second set of holes 38 in the second polyimidelayer 36 exposes selected portions of the first layer of goldinterconnects 34. The second layer of gold interconnects 40 is supportedby the second polyimide layer 36 and makes electrical connection withthe first layer of gold interconnects 34 through the second set of holes38. Finally, a third polyimide layer 42 insulates the second layer ofgold interconnects 40, and cutouts 44 around the four edges of thesegments 12 expose selected portions of the second layer of goldinterconnects 40, forming the segment bond pads 22. To electricallyconnect the segments 12 in the stack 10, electrically conductive epoxytraces 46 are dispensed along the edges of the stack 10 in contact withthe segment bond pads 22 on each of the segments 12. The electricallyconductive epoxy traces 46 also function to allow external electricalcircuits to address the segments 12 after the segments are programmed,as explained below.

FIG. 2 a diagram showing conductive fuses 50 of the present inventionlocated on a segment 12 that enable the segment 12 and the die 14thereon to be programmed. Every electrical signal transmitted to thesegment 12, including power and ground, must first enter the segment 12through a segment bond pad 22. Address signal A0 is shown, for instance,entering the segment 12 through segment bond pad 22, and then connectingto the four die 14 (D1, D2, D3 and D4). Before connecting to the diebond pads 20, the A0 signal must first pass through a respectiveconductive fuse 50. The second layer of gold interconnects 40 (FIG. 1)not only carry signals from the segment bond pads 22 to the die bondpads 20, but also form the conductive fuses 50.

Before segments 12 are assembled into a stack 10 and connected toexternal electrical circuits, such as A0 for example, the segments 12are first tested for functionality. According to the present invention,if one of the die 14 is found to be defective (e.g., die D3), thedefective die D3 is not physically cut out and discarded as in prior artmethods, but is rather electrically disconnected from the segment 12. Inone preferred embodiment, defective die 14 are disconnected byelectrically opening the respective conductive fuse 50 on the secondlayer of gold interconnects 40 (hereinafter gold interconnect 40) of thedefective die 14.

FIG. 3 is a diagram depicting one embodiment of the present inventionfor electrically opening a conductive fuse 50 using a circuit 60. Asshown, the conductive fuse 50 is formed from a generally rectangularportion of gold interconnect 40, although other shapes are alsosuitable. The circuit 60 used to open the conductive fuse 50 includes acharged capacitor 62 which is located between two probes 64 and 66. Oncethe probes 64 and 66 contact the conductive fuse 50, the capacitor 62applies a positive voltage to probe 64 and a negative voltage to probe66. The resulting current created between the probes 64 and 66 is largerthan the capacity of the conducting fuse 50 and causes the conductingfuse 50 to physically open.

One disadvantage with the structure of the conductive fuse 50 shown inFIG. 3 is that during the process of opening the conductive fuse 50, thecurrent created by the capacitor 62 often times escapes across the goldinterconnect 40 and damages the die circuitry. Another disadvantage withthe structure of the conductive fuse 50 is that not every attempt toopen a conductive fuse 50 by the circuit 60 will succeed, because insome instances, the voltage generated by the circuit 60 is insufficientto overcome the capacitance of the conductive fuse 50. Thus, defectivedie may unintentionally remain electrically connected to a segment andresult in a segment that may not function properly.

FIG. 4A is a diagram showing a second embodiment of the conductive fuse50 of the present invention. The conductive fuse 50 includes a dual fusestructure wherein fuse 72 is connected in series with fuse 74. Becausefuses 72 and 74 are smaller in area than the surrounding areas of theconductive fuse 50, the fuses 72 and 74 also have less electricalresistance and require less voltage to be opened.

FIG. 4B is a diagram showing one method for opening the conductive fuse50 of FIG. 4A, using a three probe circuit 80. The three probe circuit80 includes two outer probes 82 and 84 and one center probe 86. A firstcapacitor 88 is located between the outer probe 82 and the center probe86, and a second capacitor 90 is located between the center probe 86 andthe outer probe 84. Once the three probes 82, 84, and 86 contact theconductive fuse 50, a negative charge is applied to the two outer probes82 and 84 while a positive charge is applied to the center probe 86. Thecurrent generated across the conductive fuse 50 is greater than thecapacitance of fuses 72 and 74, and causes both fuse 72 and fuse 74 tophysically open.

The three probe circuit 80 and the dual fuse structure of the conductivefuse 50 provide several advantages. One advantage is that the threeprobe structure of the circuit 80 prevents the current from escapingacross the gold interconnect 40 and damaging nearby circuits because thecurrent created by the capacitors 88 and 90 is trapped between the twoouter probes 82 and 84. Another advantage is that the dual fusestructure of the conductive fuse 50 increases the likelihood that theconductive fuse 50 will be opened by the current produced by the threeprobe circuit 80. Typically, the current generated by the three probecircuit 80 is sufficient to disconnect a defective die 14 by openingboth fuse 72 and fuse 74. In some instances, however, the three probecircuit 80 may only open either fuse 72 or fuse 74. But even where onlyone of the two fuses 72 and 74 is opened, a defective die 14 is stilldisconnected from a segment 12 since the open fuse 72 or 74 prevents anycurrent from reaching the defective die 14. Thus, the two fuse structureof the conductive fuse 50 increases the chances that the conductive fuse50 will be opened to disconnect a die 14, thereby increasing the yieldrate for segments 12 possessing the desired functionality.

FIG. 5 is a diagram of a third embodiment of opening a conductive fuse50. In this embodiment, defective die 14 (FIG. 2) are disconnected usinga laser 92 to vaporize and open the center portion of a conductive fuse50. Since the energy from the laser 92 is sufficient to penetrate thegold interconnect 40, which forms the conductive fuse 50, the laser 92is also capable of destroying the circuitry of the die 14 on a segment12. To prevent the laser 92 from destroying the circuitry on the die 14,the first layer of gold interconnects 34 is patterned on the segment 12underneath each conductive fuse 50 into a heat shield 94 to protect thedie 14 from the laser 92. The width of the heat shield 94 issufficiently larger than the width of the conductive fuse 50 so that thelaser 92 may be activated at a point inside the heat shield 94, butoutside the conductive fuse 50. The energy of the laser 92 is adjustedso it vaporizes the top layer of gold interconnects 40, but not the heatshield 94 underneath.

Referring again to FIG. 1, after the defective die 14 are disconnected,the segments 12 are programmed so that external decoding circuitry mayaccess each segment 12 in the stack 10. For the purposes of thisdisclosure, programming refers to the process of routing control linesso that redundant functional die 14 replace the disconnected defectivedie 14. This is accomplished by connecting the control lines originallyintended for the disconnected die 14 to the replacement die 14.Programming is necessary because once the segments 12 are stacked andbecome operative, a computer or the like may attempt to access adisconnected die 14 in the stack 10. Therefore, the segments 12 thathave defective die 14 must be programmed so that when an attempt is madeto access a defective die 14 in a stack 10, a functioning die 14 isaccessed instead.

The term programming encompasses two separate programming procedures,segment level programming and circuit board level programming. Segmentlevel programming, which occurs during the fabrication of a stack 10,refers to the process in which each segment 12 in a stack is made uniquewith respect to one another. Circuit board level programming, whichoccurs once a stack 10 is connected to external circuits, refers to theprocess of logically replacing defective die 14 in the stack 10 withfunctioning die 14 in the stack 10, described further below.

Referring now to FIG. 6, a segment 12 having four die 14 (D1, D2, D3,and D4) is shown, on which segment level programming has been performedaccording to the present invention. Although only shown on die D1 forpurposes of illustration, each die on the segment 12 is externallyaddressed through a conventional row address strobe (RAS) signal 98. Theinterface between the RAS signal 98 and a die 14 comprises a set ofsegment control-bond-pads 100 and corresponding die control-bond-pads102 equal to the number of levels in the stack. Each individual set ofcontrol-bond-pads 100 and 102 is assigned to a different level in thestack. Assuming a stack of segments 12 contains five levels, forexample, then a series of five segment control-bond-pads 100 andcorresponding die control-bond-pads 102 are required on each die 14,shown in FIG. 6, as L1, L2, L3, L4, and L5.

During segment level programming, each segment 12 in a stack 10 is madeunique with respect to one another by leaving intact the conductive fuse50 assigned to the level that the segment 12 occupies in the stack 10.The conductive fuses 50 assigned to other levels in the stack 10 areopened. If, for example, the segment 12 in FIG. 6 occupies level two ina stack 10, then all the fuses 50 would be opened (using the methodsdescribed above), except the conductive fuse 50 assigned to level L2, asshown. Similarly, the segment 12 occupying level three (not shown) inthe stack 10 would have all the conductive fuses 50 opened except theconductive fuse 50 assigned to level L3, and so on for each level in thestack 10. When the RAS signal 98 is subsequently activated, only thesegment 12 whose conductive fuses 50 are intact for that level willreceive the RAS signal 98.

After segment level programming is performed on the segments 12, thesegments 12 are stacked, and the stack 10 is electrically connected toexternal circuits, typically located on a circuit board for example. Ina preferred embodiment of the present invention, the stack 10 issubsurface mounted to a circuit board, rather than mounted on thesurface of the circuit board, and electrical contact is made between thestack 10 and the circuits on the circuit board through the use ofelectrically conductive epoxy.

Referring to FIG. 7, a circuit board 150 is shown with a stack 10subsurface mounted therein. The stack 10 is placed in a hole 154 cutinto the circuit board 150 so that the top segment 12 of the stack 10 iscoplaner with the surface of the printed circuit board 150, as shown.The stack 10 is held in place by small drops of fast-curing positionalepoxy 158 applied at various locations around the perimeter of the stack10.

The circuit board 150 includes a plurality of metal traces 160 whichrepresent control signals. The stack 10 is positioned in the circuitboard 150 so that the segment control-bond-pads 22 around the perimeterof the top segment 12 match the positions of the metal traces 160 on thecircuit board 150. To bridge the gap between the segmentcontrol-bond-pads 22 and the metal traces 160 on the circuit board 150,a dispense mechanism 132 applies silver filled conductive epoxy betweeneach segment control-bond-pad 22 and an opposing metal trace 160 on thecircuit board 150 forming epoxy whiskers 162.

The epoxy whiskers 162 along with the epoxy traces 46 (FIG. 1), whichare dispensed along the edges of the stack 10, interconnect the segments12 in the stack 10 and provide an electrical connection between thecircuit board 150 and the stack 10. Referring to FIGS. 1, 6 and 7, theRAS signal 98 originates from the circuit board 150 in the form of metaltraces 160 and extends to each segment 12 in the stack 10 via the epoxywhiskers 162 and epoxy traces 64. Using the RAS signal 98 and conductivefuses 50, segment level programming of the present invention enablescircuitry on the circuit board 150 to access any segment 12 in the stack10.

FIG. 8 is a perspective view of a stack 10, which includes five levelsof segments 12, labeled L1, L2, L3, L4, and L5, where each segment 12includes four die 14 (D1, D2, D3, and D4). After the stack 10 has beenfabricated and mounted onto a circuit board 150, it is not uncommon forsemiconductor devices, such as die 14, to fail after a short period ofoperation. When a defective die 180 (D1) within a segment 12 of thestack 10 fails, the defective die 180 cannot be physically removedwithout damaging both the segment 12 and the stack 10. Die 14 failuresare therefore remedied by logically replacing the defective die 180 witha functioning die 14 in the stack 10 through circuit board levelprogramming.

For purposes of illustration, assume that every die 14 on levels L1through L4 in the stack 10 shown in FIG. 8 are required to produce afunctioning electronic device. As stated above, however, die 180 onlevel L3 is defective. Since the die 14 on L1 through L4 are unavailableto replace the defective die 180, level L5 is provided as a redundantlevel to supply replacement die 14. On a redundant level, only areplacement die 14 that is located in the same vertical column in thestack 10 as a defective die 180 is used to replace the defective die180. Therefore, only the replacement die 182 on the redundant level L5may be used to replace the defective die 180 on level L3 because bothdie 180 and 182 are in the same vertical column in the stack 10.

To logically replace the defective die 180 with the replacement die 182,the defective die 180 is first electrically disconnected from thesegment 12, as described above. Next, the RAS signal 98 (FIG. 6) of thedefective die 180 is routed to the replacement die 182 in accordancewith the present invention.

FIG. 9 is a top view of a segment 12 in a stack 10 and a circuit board150, showing how segments 12 are addressed from the circuit board 150 toallow the logical replacement of die 14 in the stack 10. The segment 12,which as an example is shown occupying level three (L3) in the stack 10,is addressed from the circuit board 150 through a set of RAS lines 200,and a set of metal switches 202. The set of RAS lines 200 includes aseparate RAS line for each level in a stack 10, plus a line indicatingno signal, called OFF. Thus, a stack 10 having five levels require sixlines, one for each level in the stack and one for off, shown in FIG. 9as RAS L1, RAS L2, RAS L3, RAS L4, RAS L5, and OFF.

As stated above, each die 14 in the stack 10 is addressed through aseparate RAS signal 98. Connected between each RAS signal 98 and the RASL1, RAS L2, RAS L3, RAS L4, RAS L5, and OFF lines, is a metal switch202. Although only one RAS signal 98 and one metal switch 202 is shownfor die D1 in FIG. 9, it is to be understood that every die 14 on eachlevel in the stack 10 is connected to a RAS signal 98 and acorresponding metal switch 202.

The purpose of the metal switch 202 is to provide a flexible method forrouting one of the RAS lines 200 to a RAS signal 98. In a preferredembodiment, the metal switch 202 is made of copper, although otherconductive metals are also suitable. The metal switch 202 is capable ofrouting any of the RAS lines 200 to a corresponding RAS signal 98through the use of an epoxy whisker 162 which is applied between themetal switch 202 and the selected RAS line 200.

During circuit board programming, every die 14 on a segment 12 in astack 10 is connected to the RAS line 200 assigned to the level thesegment 12 occupies. As shown in FIG. 9, since the segment 12 occupieslevel L3 in the stack, each die 14 on the segment 12 is connected to theRAS L3 line by dispensing an epoxy whisker 162 between each metal switch202 and the RAS L3 line. When a die 14 is found to be defective, thenthe defective 14 is logically replaced with a functioning die 14 byrerouting the RAS line 200 originally assigned to the defective die 14to a redundant functioning die 14 using each of the die's respectivemetal switches 202.

FIGS. 10A and 10B illustrate how the metal switches 202 are utilizedduring circuit board programming to logically replace a defective die180 in a stack 10 with a functioning die 182. As in FIG. 8, the stack 10shown contains five levels (L1 through L5) where level L5 is redundant.Level L3 in the stack 10 contains the defective die 180 which must belogically replaced with the replacement die 182 on level L5. FIG. 10Ashows how the die 14 on both levels L3 and L5 are addressed in the stack10 before it is discovered that die 180 is defective. FIG. 10B shows howthe defective die 180 on level L3 and the replacement die 182 on levelL5 are addressed after circuit board programming.

Referring to FIG. 10A, the segment 12 on level L3 is addressed asdescribed with reference to FIG. 9. The die 180 is originally connectedto the RAS L3 line by an epoxy whisker 162A that is connected betweenthe RAS L3 line and the metal switch 202A. Also, the RAS signal 98A isconnected between the metal switch 202A and the defective die 180through the conductive fuse 50A.

As shown in FIG. 10A, level L5 is not originally connected to a RAS line200, since level L5 in the stack 10 is redundant and is not required tobe addressed from the circuit board 150. Also, the die 14 on level L5are assigned to level L5 in the stack during segment level programmingby connecting the RAS signal 98B to the conductive fuse 50B assigned tolevel L5, and opening the remaining fuses.

As shown in FIG. 10B, after it is discovered that die 180 is defective,circuit board programming is used to replace the defective die 180 inlevel L3 with the replacement die 182 in level L5. The first step incircuit board programming is to electrically disconnect the defectivedie 180 from level L3. In a preferred embodiment, the defective die 180is first electrically disconnected from level L3 by moving the epoxywhisker 162A from the metal switch 202A and the RAS L3 line and placingthe epoxy whisker 162A between the metal switch 202A and the OFF line.Alternatively, the defective die 180 may be electrically disconnectedfrom level L3 by opening the fuse 50A (see FIG. 10A) assigned to levelL3.

After the defective die 180 is disconnected from level L3, the RAS L3line is re-routed to the replacement die 182 in level L5 by dispensingan epoxy whisker 162B between the metal switch 202B and the RAS L3 line.Since the RAS L3 line is now routed to the replacement die 182 throughthe metal switch 202B and the RAS 98B signal, when an attempt issubsequently made to access the defective die 180 through the RAS L3line, the replacement die 182 will be accessed instead. The remainingdie 14 on level L5 may be used to replace other defective die found onany level within the stack 10, as described above.

In the examples above, it was assumed that a functioning electronicdevice required a stack of four functioning segments. In a preferredembodiment, where a device requires a stack of four segments, the stackis fabricated so that four redundant levels of segments may be added tothe stack, providing a stack of eight segments. Creating a stack havingequal numbers of redundant segments as required segments provides afunctioning electronic device where only 50% of the die in the stackfunction properly.

In sum, a method and apparatus has been disclosed for segment levelprogramming and circuit board level programming a stack of segments.Through the use of conductive fuses, metal connectors, and conductiveepoxy, the programming methods of the present invention enable externalcircuits to uniquely address segments within a stack. In addition, theprogramming methods and apparatus of the present invention may also beused to uniquely address individual chips, rather than segments.

The foregoing descriptions of specific embodiments of the presentinvention have been presented for purposes of illustration anddescription. They are not intended to be exhaustive or to limit theinvention to the precise forms disclosed, and it should be understoodthat many modifications and variation are possible in light of the aboveteaching. The embodiments were chosen and described in order to bestexplain the principles of the invention and its practical application,to thereby enable others skilled in the art to best utilize theinvention and various embodiments with various modifications as aresuited to the particular use contemplated. It is intended that the scopeof the invention be defined by the claims appended hereto and theirequivalents.

What is claimed is:
 1. An apparatus for routing a set of control-signallines to a plurality of die located on a multi-level stack of a siliconsegments, said apparatus comprising;a plurality of bond pads on saiddie; a plurality of electrically conductive fuses connected to saidplurality of bond pads; a signal line located externally of saidsegments electrically connected to said plurality of electricallyconductive fuses; switch means electrically connected between saidsignal line and said set of control-signal lines for routing aparticular one of said control-signal lines to said die; means forelectrically disconnecting said die from said segments, and wherein saidmulti-level stack of segments include said plurality of dieinterconnected through metal interconnects, said plurality ofelectrically conductive fuses being formed from said metalinterconnects.
 2. An apparatus as in claim 1 wherein said segmentincludes a plurality of segment bond-pads, and said signal line enterssaid segment through one of said plurality of segment bond-pads.
 3. Anapparatus as in claim 1 wherein said switch means comprises a metalswitch and electrically conductive epoxy removably placed between saidmetal switch and a particular one of said control-signal lines.
 4. Anapparatus as in claim 3 wherein said means for electricallydisconnecting said die includes an electrical circuit capable of openingsaid electrically conductive fuses located on said die.
 5. An apparatusas in claim 4 wherein said means for electrically disconnecting said dieincludes a laser capable of opening said electrically conductive fuseslocated on said die.
 6. An apparatus as in claim 5 wherein said set ofcontrol-signal lines includes an off-signal line, and said means forelectrically disconnecting said die uses said switch means to connectsaid die to said off-signal line.